Using a new inducible form of phosphatidylinositol 3-kinase (PI 3-kinase) we have found that PI 3-kinase activation has the following effects on cell growth and proliferation. (i) Activation of PI 3-kinase was sufficient to promote entry into S phase of the cell cycle within several hours. This was shown by activation of cyclin-dependent kinase 4 (Cdk4) and Cdk2 and by the induction of DNA synthesis. (ii) PI 3-kinase activation alone was not, however, sufficient to provide for progression through the entire cell cycle. Instead, prolonged activation of PI 3-kinase in the absence of serum stimulation resulted in apoptosis. It is possible that the cells undergo apoptosis because the PI 3-kinase-induced entry into the cell cycle is abnormal. For example, we found that the cyclin E-Cdk2 complex, which normally disappears after entry into S phase of the cell cycle, fails to be downregulated following induction by PI 3-kinase. (iii) Finally, we found that prolonged activation of PI 3-kinase in the presence of serum resulted in cellular changes that resemble those associated with oncogenic transformation. The cells reached high densities, were irregular and refractile in appearance, and formed colonies in soft agar. In contrast, neither PI 3-kinase nor serum stimulation alone could induce these changes. Our results suggest that activation of PI 3-kinase promotes anchorage-independent cell growth and entry into the cell cycle but does not abrogate the growth factor requirement for cell proliferation.Phosphatidylinositol (PI) 3-kinase has been shown to mediate signaling induced by numerous growth factors and tumor antigens. The intracellular levels of the phospholipid products of PI 3-kinase increase in response to stimulation with growth factors or after oncogenic transformation (for reviews, see references 10,11,33,76,80). PI 3-kinase signaling appears to be required for a number of mitogens during the G 1 -to-Sphase transition of the cell cycle (63). Recently, it was demonstrated that PI 3-kinase regulates cell survival in response to various apoptotic stimuli (21, 49).PI 3-kinase is a heterodimeric complex consisting of an 85-kDa regulatory subunit, p85, and a 110-kDa catalytic subunit, p110 (11, 33). The p85 subunit contains two Src homology 2 (SH2) domains, which bind to tyrosine-phosphorylated receptors after stimulation of cells with growth factors and in this manner recruit the p85-p110 complex to the cell membrane. The region between the two SH2 domains of p85, the iSH2 region, mediates the association with p110, and this interaction is required for the enzymatic activity of p110 (37). Based on this observation we generated a chimeric molecule, p110*, in which the iSH2 region of p85 was covalently linked to its binding site at the p110 N terminus by using a flexible hinge region (30). p110* is a constitutively active PI 3-kinase which can activate signaling pathways independent of growth factor stimulation.The generation of constitutively active PI 3-kinase molecules has greatly facilitated the analysis of signa...

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PKR (protein kinase, interferon-responsive) is a ribosomal-associated protein kinase found in all human cells. When activated by dsRNA or polyanionic substances, PKR efficiently inhibits cellular protein synthesis. PKR expression has been correlated with cellular differentiation in a number of tumor types, including squamous cell carcinoma of the head and neck region. Although transfection of PKR into mouse fibroblasts and yeast cells inhibits proliferation, it is not known if modulation of native PKR levels occurs during cellular proliferation and differentiation in human normal and neoplastic tissues. To determine whether PKR expression was inversely related to proliferative activity in vivo, we used double-label immunohistochemistry to colocalize PKR and the proliferation marker, proliferating cell nuclear antigen (PCNA), in a series of head and neck squamous cell carcinomas. Overall, neoplasms demonstrating high levels of PKR showed low levels of PCNA immunoreactivity; carcinomas with low levels of PKR expressed high levels of PCNA. Within individual tumors, PKR and PCNA showed an inverse regional distribution: PKR was located predominantly in the center of tumor nests, while PCNA was restricted to the periphery. Patients whose tumors expressed high levels of both PKR and PCNA had the longest mean disease-free survival. These findings support the hypothesis that PKR levels are modulated in cell proliferation and differentiation in head and neck squamous cell carcinoma. Further studies are needed to clarify the mechanisms underlying the antiproliferative activity of PKR.

The eastern Mojave Desert of San Bernardino County, California is a topographically diverse region which supports a rich and varied herpetofauna. The area included in this study is hounded on the north ty Interstate Highway 15 and on the south hy Route 66 . A line from the town of Ludlow to Soda Dry Lake forms the western edge, while the Nevada state line is the boundary on the east. Elevations within this area range from about 900 feet above sea level at Soda Dry Lake to over 67 00 feet in the Granite Mountains. Even higher mountains lie just outside the northern boundary. Playas, sand dunes, bajadas, canyons, plateaus, and mesas make up the other physiographic features of the area. Virtually the entire region lies within the Lower Sonoran and Upper Sonoran Life Zones (Merriam, I 898 ). Only the Clark and Spring Mountains just north of this area reach the Transition and Canadian Life Zones.